Occasionally situations arise that demand the use of an ultrasonic transducer in the 100-200 kHz range with minimal power requirements and operating into air or other gases. The low power requirement rules out a large number of existing transducers--whether their sensitivities are so poor that they need a large stimulating voltage or whether they need a large bias voltage which is difficult to achieve in a low power D.C. system. For example, piezoelectric ultrasonic transducers (commonly used underwater) operating into air or other gases are typically of low sensitivity or narrow bandwidth. These characteristics result from the immense acoustic impedance mismatch between air or other gases and the transduction materials (the latter being able to create large forces but with only small deflections). Either one puts up with the small deflections (low acoustic output) or one brings the material into a resonant state at one particular frequency. For echo sensing or information transmission applications a single frequency is useless and as broad a range of frequencies as possible is desirable. Some low-bias-voltage (30V) electrostatic transducers have been developed but, by and large, these are expensive and time-consuming to produce.